njit-etd2003-081 - New Jersey Institute of Technology

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The final step to obtain the power spectrum of HRV is to take the FFT of the
detrended IIBI signal. In the existing software, this is done by decimating the IIBI signal
by a factor of ten (the ECG is sampled at 200 Hz) and taking an 8192 point FFT of the
decimated IIBI signal. When the signal is decimated by a factor of ten, every tenth point
of the original signal is kept, and the nine points in between every tenth point are not
used. In a time series of samples, every tenth point occurs at the same time as in the
undecimated signal, except that there are no samples in between. In effect, decimating is
similar to down-sampling. In other words, because the length of the IIBI signal is
approximately the same length of the sampled ECG, which is acquired using a sampling
frequency of 200 Hz, decimating the IIBI by a factor of 10, is similar to sampling the
IIBI at 20 Hz. This can be done because the IIBI signal contains no frequency
components above 6 Hz.
The software is programmed to take an 8192-point FFT by default regardless of
the input signal length. In the experiments, 300 seconds of ECG were collected. At a
sampling frequency of 200 Hz, this corresponds to 60,000 samples. Recall that the IIBI
is approximately the same length as the ECG. If an 8192-point FFT were taken, only the
first one-eighth of the 1113I would be represented in the spectrum. In addition, the
frequency resolution would be 200/8192= 0.0244 Hz and the spectrum would be
between 0Hz and 100 Hz. However, the spectrum of the IIBI, the heart rate variability
spectrum, consists of low frequencies less than 6 Hz; therefore, the spectrum does not
need to be calculated up to 100 Hz. As a result, the 11131 is decimated by a factor of ten.
In this case, the IIBI signal would then be 6,000 samples long for a 300-sec long ECG
signal. Note in Figure 3.6 that the length of the IIBI signal is 6,000 points long. The